Police have searched the home of former President Jair Bolsonaro in the Brazilian capital, Brasilia, as part of an investigation into his Covid vaccination records.
Police suspect the ex-leader's vaccination record was forged so he could gain entry to the US.
Officers seized the mobile phone of the former president and that of his wife and arrested some of his close aides as part of the operation.
Mr Bolsonaro has denied any wrongdoing.
The former president is a vocal opponent of Covid vaccinations, who repeatedly spread false information about the vaccine and its alleged side effects.
From 21 April 2020: The BBC's South America correspondent Katy Watson looks at how Bolsonaro has responded to the virus in Brazil
While he had always said that he would not get the vaccine, he had refused to make his vaccination record public, arguing that it contained sensitive private data.
But an official in the new government of his successor, Luiz Inácio Lula da Silva, ruled that the information was in the public interest.
The record showed that Mr Bolsonaro had had a vaccine in 2021 but police think the record may have been forged and launched the investigation which led to Wednesday's raid.
According to federal police, "false data" was inserted into the Covid-19 vaccination records held by the Brazilian ministry of health so that the individuals whose vaccination status had been changed would be able to get the vaccination certificates necessary to enter the United States
Speaking after the raid, Mr Bolsonaro denied tampering with any records. "For my part, there was nothing falsified," he told reporters.
He also denied having had a Covid jab: "I didn't take the vaccine. Period."
The false data was allegedly inserted in the records between November and December of last year, when Mr Bolsonaro was still president.
Mr Bolsonaro travelled to Florida at the end of December, just days before his rival, Luiz Inácio Lula da Silva, was sworn into office.
The US has not commented on what visa the then-president entered the country on or what, if any, vaccination records he provided at the time.
Since his return to Brazil on 30 March, Mr Bolsonaro has had to appear before police on two occasions to testify in two separate investigations.
One is examining his alleged role in the storming of the Brazilian Congress by his supporters in January. He is also being investigated over whether he tried to illegally import and keep millions of dollars' worth of jewellery he and his wife were given by Saudi Arabia in 2019.